This invention relates generally to method and system for producing hypochlorite. More specifically, this invention relates to the generation of hypochlorite by electrolysis.
Chlorine in the form of hypochlorite was first used for disinfecting water systems in London after an outbreak of cholera in 1850. For the past century, chlorination has become the standard way to disinfect water supplies, potable water, wastewater treatment and swimming pools, for example to eliminate epidemic waterborne diseases. The traditional way to disinfect water with chlorine was through the use of chlorine gas. Transporting bulk chlorine on crowded highways and into residential areas has become a major safety concern since the transport of chlorine gas under high pressure can be very hazardous. Also, the transport of commercial hypochlorite, which is predominantly water, is very expensive. Stringent regulation of toxic gasses and accidental releases of chlorine and higher costs have caused alternative sources for chlorine to be sought for water disinfection. Production of a chlorine source, on-site, is currently the best option for obtaining a less expensive and safer source.
On-site generation has proven itself as a safe and cost efficient process for providing for the chlorine needs of water treatment facilities. On-site hypochlorite generation has been accomplished by different means in the past. The preferred on-site reaction is creating sodium hypochlorite (NaOCl) according to the following equation:
The chemical reaction is NaCl+H2O+2exe2x86x92NaOCl+H2.
One method used an alkali metal chlorate cell using solid salt as disclosed in U.S. Pat. No. 3,849,281 given to Bennett et al. U.S. Pat. No. 3,902,985 given to Raetzsch et al teaches the use of a cell with higher temperatures so that a brine feed solution can be used instead of a solid salt feed. Goto et al., in U.S Pat. No. 4,151,052, discloses a process of producing sodium hypochlorite comprising electrolyzing an aqueous solution of sodium chloride. Goto, in ""052 teach the use of at least one cooling means in or between the electrolytic cells to cool the electrolyte solution preferably between 5xc2x0 C. and 45xc2x0 C. for an increase in available chlorine.
The amount of available chlorine in the resulting solution can be found by the equation.
Amount of Available Chlorine (g/L)=2xc3x97(chlorine in NaClO)
It is the amount of available chlorine that determines the efficiency of the process. Competing reactions occur in the resulting sodium hypochlorite produced from the initial reaction. The amount of sodium hypochlorite lost to side reactions is proportional to greater concentrations of available chlorine and higher temperatures. The temperature of aqueous solutions used in the reaction has been found to make a difference in the resulting amount of sodium hypochlorite produced. Lower temperatures decrease the amount of side reactions that occur.
Murakami et al. in U.S. Pat. No. 4,495,048 discloses a three-compartment electrolytic cell using bipolar electrodes for the electrolysis of salt water. The ""048 reference teaches the use of heat exchange gaskets contained within the cell. U.S. Pat. No. 3,997,414 given to Casson et al. teaches a cooled cell chamber. Casson ""414 discloses a chamber for the circulation of an electrolytic solution comprised of a cooling chamber and a concentration chamber adjacent to the electrode assembly. The cooling and concentration chamber are connected by conduits in the cell.
In a process for the electrolysis of sea water to produce hypochlorite, Spaziante, in U.S. Pat. No. 4,488,945, suggests mixing sea water at temperatures below 9.6xc2x0 C. before electrolysis with recycled warmed hypochlorite solution to increase the temperature of the sea water mixture.
U.S. Pat. No. 5,294,307 given to Jackson discloses a cell that produces chlorine dioxide and alkali chlorates as its products from a recycled alkali hypochorate solution at temperatures preferably between 85xc2x0 C. and 95xc2x0 C.
What is needed is an on-site means of producing sodium hypochlorite at greater efficiency with reduced energy and feed product consumption.
The present method and system generates sodium hypochlorite or potassium hypochlorite, in terms of available chlorine, more efficiently. Increased efficiency is measured by an increase in the percentage of sodium or potassium chloride converted to hypochlorite during the electrolytic process and a decrease in power consumption. Higher temperatures of the solution during electrolysis and increased concentrations of available chlorine enhance the probability of a shift in kinetics to form undesirable by products. The amount of available chlorine, the desired end product, is reduced. Because of these side reactions, more sodium chloride and electric current must be consumed to produce an equal amount of available chlorine. Advantageously, during the method of this invention, the heat generated during the electrolysis process is abated by first chilling the influent water in a chiller separate from the electrolyzer assembly and then piping the chilled water to one or more inlets to the electrolyzer cells within the electrolyzer assembly. The additional water not only reduces the temperature but also dilutes the sodium chloride/sodium hypochlorite solution resulting from electrolysis. Side reactions are reduced thereby allowing greater production of available chlorine. The efficiency of the method can be increased so that the efficiency is within a range of from about 70% to about 80%. Improved efficiency translates to cost savings in terms of feed product and electric power consumption.
One preferred method and system can generate potassium hypochlorite by using a potassium salt as the starting product. A preferred system for generating sodium hypochlorite comprises an electrolyzer assembly having at least one electrolyzer cell. In another preferred system, the electrolyzer assembly can have up to 10 electrolyzer cells or tubes stacked one upon another. Each electrolyzer cell comprises compartments having unseparated anode and cathode plates. The number of anode/cathode compartments can range from about 1 compartment to about 15 compartments, preferably 4 to 12 compartments per cell. By piping chilled water to multiple inlets to the electrolyzer assembly, the heat produced during electrolysis is abated and the temperature within the electrolyzer cell is reduced, thereby reducing the side reactions, which consume available chlorine. Preferably, each electrolyzer cell within an electrolyzer assembly has at least one inlet and an outlet. Depending on its size, each tube or cell can have from 1 to 6 inlets and, in some embodiments, up to 10 inlets for receiving water to cool down the temperatures within the cell. The system can further comprise a chiller for cooling water and piping to transport the chilled water to the inlet(s) of each electrolyzer cell within the electrolyzer assembly. The salt solution for the electrolysis process is piped to the electolyzer assembly from a brine tank. Solid salt within the brine tank mixes with water piped to the brine tank from a water softener unit. The water sent to the chiller can also be softened within the water softener unit prior to being piped to the chiller.
During one preferred method for producing sodium hypochlorite, brine solution is piped from the brine tank to a first inlet in a first electrolyzer cell of an electrolyzer assembly while chilled water is simultaneously piped from a chiller to the first inlet so that the brine solution combines with the chilled water at the entry point to the cell. The chilled brine solution in the first electrolyzer cell is electrolyzed to produce sodium hypochlorite and hydrogen. The hydrogen is contained and vented to the atmosphere. In this method, the electrolyzer assembly can comprise one or more electrolyzer cells and each electrolyzer cell can have more than one inlet for receiving water. In one preferred method, the electrolyzer cell can comprise up to six inlets, each receiving additional water for diluting and chilling the sodium chloride/sodium hypochlorite solution. The hypochlorite and brine solution resulting from the electrolysis in the first electrolyzer cell is piped to an inlet in a second electrolyzer cell in the electrolyzer assembly. Simultaneously chilled water from the chiller is piped to the second inlet so that the chilled water combines with the hypochlorite and brine solution upon entering the second electrolyzer cell thereby diluting the solution and abating the heat caused by the electrolysis process. The chilled hypochlorite and brine solution in the second cell is than electrolyzed. This process is repeated until the hypochlorite and brine solution passes through all cells of the electrolyzer assembly.
In an alternative embodiment of the method for producing sodium hypochlorite, the method comprises piping water to a water softener unit to reduce the calcium and magnesium content and piping a portion of the softened water to a brine tank containing solid salt to form a brine solution comprising salt within a range of between 20 weight per cent and saturation, and piping a second portion of the softened water to a chiller.
The brine solution is piped from the brine tank to a first inlet in a first electrolyzer cell of an electrolyzer assembly while simultaneously piping chilled water from the chiller having a temperature range from about 10xc2x0 C. to about 25xc2x0 C. to the first inlet so that the brine solution combines with the chilled water, and electrolyzing the chilled brine solution in the first electrolyzer cell. The hypochlorite and brine solution resulting from electrolysis occurring in the first cell is piped to a second inlet in a second electrolyzer cell in the electrolyzer assembly while simultaneously piping chilled water from the chiller having a temperature range from about 10xc2x0 C. to about 25xc2x0 C. to the second inlet so that the chilled water combines with the hypochlorite and brine solution. Each cell can have more than one inlet, preferably up to 6 inlets. The chilled hypochlorite and brine solution is electrolyzed in the second cell.
The hypochlorite and brine solution resulting from electrolysis in the second cell is piped to a third inlet in a third electrolyzer cell of the electrolyzer assembly while simultaneously piping chilled water from the chiller having a temperature range from about 10xc2x0 C. to about 25xc2x0 C. to the third inlet so that the chilled water combines with the hypochlorite and brine solution. The resulting chilled hypochlorite and brine solution is electrolyzed in the third cell. The process is repeated until the hypochlorite and brine solution passes through all cells of the electrolyzer assembly and the effluent solution from the last cell comprises sodium hypochlorite within a range of approximately 5 g/l to approximately 15 g/l and sodium chloride within a range of approximately 12 g/l to approximately 25 g/l.